Fluid energy, or jet, mills are size reduction machines in which particles to be ground (feed particles) are accelerated in a stream of gas (compressed air or steam) and ground in a grinding chamber by their impact against each other or against a stationary surface in the grinding chamber. Different types of fluid energy mills can be categorized by their particular mode of operation. Mills may be distinguished by the location of feed particles with respect to incoming air. In the commercially available Majac jet pulverizer, produced by Majac Inc., particles are mixed with the incoming gas before introduction into the grinding chamber. In the Majac mill, two streams of mixed particles and gas are directed against each other within the grinding chamber to cause fracture. An alternative to the Majac mill configuration is to accelerate within the grinding chamber particles that are introduced from another source. An example of the latter is disclosed in U.S. Pat. No. 3,565,348 to Dickerson, et al., which shows a mill with an annular grinding chamber into which numerous gas jets inject pressurized air tangentially.
During grinding, particles that have reached the desired size must be extracted while the remaining, coarser particles continue to be ground. Therefore, mills can also be distinguished by the method used to classify the particles. This classification process can be accomplished by the circulation of the gas and particle mixture in the grinding chamber. For example, in "pancake" mills, the gas is introduced around the periphery of a cylindrical grinding chamber, short in height relative to its diameter, inducing a vorticular flow within the chamber. Coarser particles tend to the periphery, where they are ground further, while finer particles migrate to the center of the chamber where they are drawn off into a collector outlet located within, or in proximity to, the grinding chamber. Classification can also be accomplished by a separate classifier. Typically, this classifier is mechanical and features a rotating, vaned, cylindrical rotor. The air flow from the grinding chamber can only force particles below a certain size through the rotor against the centrifugal forces imposed by the rotor's rotation. The size of the particles passed varies with the rotor's speed; the faster the rotor, the smaller the particles. These particles become the mill's product. Oversized particles are returned to the grinding chamber, typically by gravity.
Yet another type of fluid energy mill is the fluidized bed jet mill in which a plurality of gas jets are mounted at the periphery of the grinding chamber and directed to a single point on the axis of the chamber. This apparatus fluidizes and circulates a bed of feed material that is continually introduced either from the top or bottom of the chamber. A grinding region is formed within the fluidized bed around the intersection of the gas jet flows; the particles impinge against each other and are fragmented within this region. A mechanical classifier is mounted at the top of the grinding chamber between the top of the fluidized bed and the entrance to the collector outlet.
The primary operating cost of jet mills is for the power used to drive the compressors that supply the pressurized gas. The efficiency with which a mill grinds a specified material to a certain size can be expressed in terms of the throughput of the mill in mass of finished material for a fixed amount of pressurized gas supplied to the mill. One mechanism proposed for enhancing grinding efficiency is the projection of particles against a plurality of fixed, planar surfaces, fracturing the particles upon impact with the surfaces. An example of this approach is U.S. Pat. No. 4,059,231 to Neu, in which a plurality of impact bars with rectangular cross sections are disposed in parallel rows within a duct, perpendicular to the direction of flow through the duct. The particles entrained in the air stream passing through the duct are fractured as they strike the impact bars. U.S. Pat. No. 4,089,472 to Siegel, et al. discloses an impact target formed of a plurality of planar impact plates of graduated sizes connected in spaced relation with central apertures through which a particle stream can flow to reach successive plates. The impact target is interposed between two opposing fluid particle streams, such as in the grinding chamber of a Majac mill.
Although fluidized bed jet mills can be used to grind a variety of particles, they are particularly suited to grinding toner materials used in electrostatographic reproducing processes. These toner materials can be used to form either two component developers (typically with a coarser powder of coated magnetic carrier material to provide charging and transport for the toner) or single component developers (in which the toner itself has sufficient magnetic and charging properties that carrier particles are not required). The single component toners are composed of resin and a pigment such as commercially available MAPICO Black or BL 220 magnetite. Compositions for two component developers are disclosed in U.S. Pat. Nos. 4,935,326 and 4,937,166 to Creatura, et al.
The toners are typically melt compounded into sheets or pellets and processed in a hammer mill to a mean particle size of between of 400 to 800 .mu.m. They are then ground in the fluid energy mill to a mean particle size of between 3 and 30 .mu.m. Such toners have a relatively low density, with a specific gravity of approximately 1.7 for single component and 1.1 for two component toner. They also have a low glass transition temperature, typically less than 70.degree. C. The toner particles will tend to deform and agglomerate if the temperature of the grinding chamber exceeds the glass transition temperature.
Although the fluidized bed mill is satisfactory, it could be enhanced to provide a significant improvement in grinding efficiency. The Siegel and Neu disclosures are directed to mills in which the particles are mixed in the gas jet flows outside the grinding chamber and as such are not suited for use in a fluidized bed mill. Furthermore, where flat surfaces are employed as targets, complex structural elements may be required to insure maximum exposure to the moving particles. Thus, there is a need for a mechanism to enhance the grinding efficiency of a fluidized bed jet mill.